1. Field of the Invention
The present invention relates to an oxygen sensor and, more particularly to an improvement of a plate oxygen sensor element which is used to detect the concentration of oxygen in gas, particularly in the gas exhausted from an internal combustion engine.
2. Description of the Prior Art
There have been known oxygen sensors employed as an oxygen concentration detector. These oxygen sensors perform purification of exhausted gas and economization on fuel consumption and the like by detecting the oxygen concentration in exhausted gas from an internal-combustion engine, and by suitably controlling combustion conditions of the internal-combustion engine based on the detection signals regarding the oxygen concentration.
One of these sensors employs a sensor element comprising a partition of a solid electrolyte having oxygen ion conductivity, such as zirconium oxide doped with calcium oxide, yttrium oxide or the like, the both surfaces of the partition being provided with respective electrodes. One of the electrodes is exposed to reference atmosphere and the other is exposed to gas to be measured, such as exhaust gas and the like. With this arrangement, electromotive force obtained through applying the theory of oxygen concentration cell is employed as the detection signal.
Among such oxygen detecting elements, nowadays there has been noted an elongated plate detecting element instead of a cylindrical detecting element with a bottom attached, for ease of fabrication, volume reduction and the like, either end of the plate being provided with an oxygen detection section to be exposed to gas to be measured, such as exhaust gas and the like. Japanese Patent Laid-Open Nos. 58-153155 and 61-97562 show examples of such plate oxygen detecting elements.
There has been known another type of an oxygen sensor employing a sensor element in which an elongated plate substrate is provided with an oxide at the surface, the oxide varying electrical resistance depending upon oxygen concentration, such as titanium oxide. With this arrangement, the sensor element is exposed to gas to be measured, such as exhaust gas and the like, in order to detect the variation of electrical resistance caused by partial pressure of oxygen.
FIG. 4 illustrates an example of such plate oxygen sensor element.
Such oxygen sensor element 12, as illustrated in FIG. 4, has an oxygen detection section 5 which is exposed to gas to be measured, such as exhaust gas having generally high temperature. In order that the oxygen detecting capacity of the oxygen detection section 5 in such oxygen sensor element 12 may sufficiently function, in some cases, the oxygen detection section 5 is maintained at a high temperature by a heater built in the oxygen sensor element 12. Consequently, the oxygen sensor element 12 may undergo thermal stress.
This oxygen sensor element 12 is, however, in the form of a plate; therefore, there arises stress concentration at the respective ridges 6 thereof so that cracks occur and the oxygen concentration detecting function by the sensor element 12 is damaged.